In general, when preparing an emulsion, a nonionic surfactant or an ionic surfactant is used as an emulsifier. However, since oils to be emulsified show various polarities, in order to obtain a stable emulsion, the degree of polarity of the oil, that is, the required HLB, is first obtained, and the above surfactant is selected and used accordingly. In many cases, as an emulsifier, a combination of a hydrophilic emulsifier having a high HLB and a lipophilic emulsifier having a low HLB is used.
Examples of hydrophilic emulsifiers having a high HLB include, for example, anionic surfactants such as fatty acid soap and alkyl sulfate ester salts; cationic surfactants such as distearyl dimethyl ammonium chloride and stearyl trimethyl ammonium chloride; and nonionic surfactants such as polyoxyethylene alkyl ethers, polyoxyethylene fatty acid esters and polyoxyethylene sorbitan fatty acid esters having a polyoxyethylene chain with long chain length.
Examples of lipophilic emulsifiers having a low HLB include, for example, nonionic surfactants having a short polyoxyethylene chain length, and nonionic surfactants such as sorbitan fatty acid esters and glycerin fatty acid esters, etc.
The required HLB of an oil to be emulsified is obtained by using a nonionic surfactant wherein HLB has been already known, and conventionally, this has required very complicated means such as changing the amount ratio of high HLB surfactant and low HLB surfactant. Then, an emulsion is prepared by selecting an emulsifier on the basis of the obtained required HLB; however, in fact a stable emulsion is rarely obtained, and there has been a problem that the experiment has to be repeated in a trial and error-like manner.
To solve such a problem, it is disclosed that an emulsifier containing an alkanolamide of oleic acid and an anionic surfactant can exert an emulsifying power for a relatively wide range of required HLB (Patent Document 1). However, the emulsion composition using this surfactant has a problem of high irritation to the skin.
In addition, a novel complex consisting of an amphoteric surfactant and/or a semipolar surfactant (hereinafter referred to as amphoteric surfactant) and a higher fatty acid has been proposed; however, the odor of oxidative degradation of higher fatty acids becomes a problem, and in terms of usability, since an active agent is used, stickiness derived from the active agent is generated; and this novel complex is not actually put into practical use as a basic skeleton of preparations (Patent Document 2).
In the end, an emulsifier capable of exerting excellent emulsifying power even on oils having a wide range of required HLB, and capable of achieving both stability and usability have not yet been obtained.
Incidentally, a secondary amide including an amide alcohol has been studied as an agent imparting moisture resistance, and in a composition using this, formulation wherein a secondary amide is used together with a hydrophilic acrylic polymer (a carboxyl group-containing polymer) as a thickener has been prepared (Patent Document 3). However, according to the preparation method described in this document, no complex is formed and the use of a common emulsifier (surfactant) is required for the preparation of an emulsion composition.
It has been proposed to mix polyethylene and N-erucyl-6-hydroxycaproamide to form a film (Patent Document 4). However, it is not a technique concerning the preparation of an emulsion.
It has also been proposed to prepare a silver halide color photographic light-sensitive material by mixing an alcohol including an amide alcohol with an aqueous gelatin solution comprising a surfactant to prepare an emulsion (Patent Document 5). However, it has not been proposed to carry out emulsification by using an amide alcohol and a carboxyl group-containing polymer.